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Unformatted text preview: The ﬂux of material through a plane depends on the concentration diﬀerence
J = −D dC
dx where D is the diﬀusion constant
dx (45.2) This is Fick’s ﬁrst law of diﬀusion (in one dimension).
The change in concentration in a lamina between x and dx with time is given by
the ﬂux in minus the ﬂux out of the lamina:
J (x) − J (x + dx)
∂x (45.3) Using Fick’s ﬁrst law for J
If D is truly constant we get Fick’s second law of diﬀusion:
∂x (45.4) (45.5) 302 ...
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- Spring '09